Braking system with nonbackdriveable actuator

ABSTRACT

A vehicle braking system is provided. In a preferred embodiment, the system has an actuator frame with a bore fluidly connected to the wheel brake and to a master cylinder. A piston, mounted within the bore, is operationally associated with a non-rotative nut which is threadably engaged by a power screw. To maximize reaction speed, a high efficiency thread is utilized between the power screw and the non-rotative nut. Therefore, pressure within the actuator can backdrive the piston. To prevent backdriving of the piston, there is a spring locking mechanism. The spring locking mechanism has a driver, a driven member and a coil spring with a tang at its end. The driven member has an elastomeric torsion member to absorb torsion shocks.

The present invention is a Division of U.S. Ser. No. 07/694,286, filedMay 1, 1991 which is a continuation-in-part of U.S. Pat. No. 5,011,237.

FIELD OF THE PRESENT INVENTION

The field of the present invention is that of anti-lock braking systems(ABS) for automotive vehicles.

DISCLOSURE STATEMENT

Anti-lock braking systems typically modulate the pressure delivered to avehicle wheel brake to prevent the vehicle wheel from locking up in thebraking condition. Two prior anti-lock braking systems are shown incommonly assigned U.S. Pat. Nos. 4,653,815 and 4,756,391. In both of theaforementioned systems, an electronic controller signals a motor whichis gearably engaged with a driven member which is in turn threadablyengaged with an actuator piston, piston movement is used to modulate thepressure delivered to the vehicle wheel brake.

SUMMARY OF THE INVENTION

The present invention provides a vehicle anti-lock braking system whichis an alternative to the aforementioned anti-lock braking systems. Toreduce the space envelope of the aforementioned systems, the present ina preferred embodiment invention provides a piston which is attached toa non-rotative nut which is threadably engaged by a power screw ratherthan having the piston attached to a non-rotative screw which is engagedby a rotative nut. The above modification allows the present inventionto provide anti-lock braking systems wherein the components are smallerand wherein there is less rotative mass, thereby greatly reducing theangular inertia of the system.

Furthermore, in a preferred embodiment, the present invention providesan actuator with a check valve providing an alternative flow patternfrom the master cylinder to the wheel brake (cylinder). The check valveis opened by the piston itself. Therefore, there is an alternative flowpath to the wheel brake.

To maximize reaction speed, a high efficiency, low friction thread isutilized between the power screw and the non-rotative nut. Therefore,pressure within the actuator can back drive the piston. In the preferredembodiment the present invention is configured in such a manner and thatthe piston is at its extreme position opens the check valve when thesystem is not in the ABS mode of operation. The piston can be held withan inefficient screw. However, it has been found preferable to use anefficient screw (to lower the current required by the motor). Therefore,to make the present invention work, there must be some means of holdingthe piston at its extreme position when it is exposed to master cylinderpressure, but not within the ABS mode wherein the motor is beingpowered. Space limitations under the hood of an auto require that thesolution to the above need take up as little space as possible.

One method to prevent back drive of the piston is to use anelectromagnetic brake which restrains movement of the motor. Such ananti-lock braking system is disclosed in U.S. Pat. No. 5,000,523 toMikhaeil-Boules et al. The present invention provides an anti-lockbraking system which is an alternative to that found in Mikhaeil.

It is an object of the present invention to provide an anti-lock brakingsystem.

Other objects and advantages of the present invention can become moreapparent to those skilled in the art as the nature of the invention isbetter understood from the accompanying drawings and a detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view show partially in section of a preferredembodiment of the present invention for one vehicle wheel brake;

FIG. 2 is a sectional view illustrating details of an anti-lock brakingsystem illustrated in FIG. 1;

FIGS. 3 and 4 are views taken along lines 3--3 and 4--4 respectively ofFIG. 2;

FIG. 5 is a view illustrating the driver rotated from the position shownin FIG. 4;

FIG. 6 is an exploded view of the spring driver and pinion gear utilizedin the anti-lock braking system of the present invention;

FIGS. 7, 8 AND 9 are views taken along lines 7--7, 8--8 and 9--9,respectively, of FIG. 6;

FIG. 10 is a view taken along line 10--10 of FIG. 9;

FIG. 11 is a side elevational view of the driver illustrated in FIGS. 2,6, 9 and 10.

FIGS. 12, 13, 14, 15, 16, 17, 18, 19, 20 and 21 illustrate alternativepreferred embodiments of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The vehicle wheel anti-lock braking system 7 of the present inventionincludes a master cylinder 12 for supplying pressurized fluid. Connectedon the wheel 14 and schematically shown, is a fluid activated wheelbrake cylinder 16 (hereinafter referred to as a wheel brake) whichreceives pressurized fluid from the master cylinder for restrainingrotational movement of the wheel 14. The wheel brake 16 may be utilizedin a conventional drum or disc type vehicle brake.

An ABS electronic controller 18 is also provided. A sensor 20 in thevehicle wheel brake 16, determines the wheel 14 rotational speed and asensor (not shown) determines whether or not the brake pedal 22 of thevehicle is activated. Both sensors feed information to the ABScontroller 18. The ABS controller 18 will be cognizant of the rotationalcondition of the wheel and will provide an appropriate signal inresponse thereto. The signal will place the brake system in an ABS modeof operation if the condition of the wheel 14 is within presetparameters.

A normally open solenoid valve 24, when activated to a closed positionin response to a signal given by the controller 18, functions as anisolation valve to prevent fluid communication between the mastercylinder 12 and the wheel brake 16. An actuator 28 is provided having anactuator frame 30 with a longitudinal bore 32. An actuator can beprovided for each wheel brake of the vehicle or if desired, a pluralityof wheel brakes can be connected to a single actuator. The longitudinalbore 32 has a first fluid connection 42 allowing fluid communicationwith the wheel brake 16 and the longitudinal bore 32 also has fluidcommunication with the master cylinder 12 when the solenoid valve 24 isnot activated to the closed position via passage 40. Additionally, asshown, the longitudinal bore has a second or alternative fluidcommunicative path with the master cylinder 12. As shown, the bore 32 ismidstream of the solenoid valve 24 and passages 42. Fluid flow passesover a transverse slot (not shown) of a piston 44. However, the solenoidvalve 24 could directly tie into the wheel brake 16 and passage 42 could"T" into that line. The alternative path 34 has a check valve 38 whosefunction will be described later. The check valve 38 allows delivery offluid back to the master cylinder 12 whenever the wheel brake 16 has apressure greater than that in the master cylinder 16. Therefore, thebraking system is sensitive to an operator relieving the brake byremoving his or her foot therefrom without any needed input from thecontroller.

The piston 44 is slidably and sealably mounted within the longitudinalbore 32. Movement of the piston 44 provides a variable control volume incommunication with the wheel brake 16, thereby modulating the pressuretherein. A nut 46 operatively associated with piston 44 is connectedwith the piston 44 and the nut 46 is slidably mounted within thelongitudinal bore 32 in a non-rotative fashion.

A power screw 48 projects into the nut and is threadably engagedtherewith in an efficient manner. The power screw has a fixed rotationalaxis with respect to the actuator frame 30. Powering the power screw isa reversible DC motor 50 which is responsive to the signals given to itby the controller 18. In the position shown, for normal brakingoperation, the piston 44 is held at the extreme up position and must beheld within a tolerance of 3/100 of an inch to maintain the check valve38 in the open position via the rod 52 (tolerance shown in FIG. 1greatly enlarged for purposes of illustration).

The power screw 48 is connected to a gear train 80 which is in turnconnected also with the motor 50. The power screw is mounted by bearingsand has a end large gear 82 connected to the end thereto. The large gear82 meshes with an idler gear 84 which in turn meshes with a smallerpinion gear 86. The pinion gear 86 axially floats on a rotor shaft 88 ofthe motor and is held on by a spring clip (not shown). Fixably connectedto the rotor shaft 88 projecting away from the motor is a driver member100 (herein after referred to as the driver). The driver 100 has aflange portion 110 towards the end of the motor and is symmetrical androtationally balanced. The driver 100 also has four tang contactsurfaces 104 whose function will be described later. The driver 100 hasan angular position which corresponds with the position of the rotorshaft 88.

Generally surrounding the driver 100 and located between the motor andthe power screw (in the example shown) adjacent to the DC motor is asleeve 120 having a location generally fixed with respect to the motor50.

The sleeve 120 has a generally smooth sliding surface on the interiorand has an inner diameter of a first value. A spring 130 which has afree outer diameter of a second value equal to or greater than saidfirst value is captured within the non-rotative sleeve 120. The coilspring 130 at both ends has a generally radially inward, projecting tang132 with a radius curvature equal to the spring thickness. The tangs 132will project inward intersecting a line tangent with the sleeve 120 atangle less than 90 degrees and at their inner most radial portion 134have a generally straight section 136. Typically, the spring will bemade with piano wire with a square or rectangular cross-sectional shapedarea.

The motor's housing 56 has a three diameter inner bore. In the inwardinner diameter of the bore a ball bearing 133 is press-fitted therein.The ball bearing 133 mounts the rotor shaft 88. The rotor shaft 88projects outwards and has the driver 100 keyed, splined, or pressedfitted to it the driver 100.

Separated by a landing 135 in the housing inner bore from the bearing133 and press-fitted in its third interior diameter is the sleeve 120.

Lockably engaged with the sleeve is the spring 130 with its two radiallyinward facing tangs 132. The tangs are configured in such a manner thatrotation of the driver causes the curved driver tang contact surface 104to pull the tangs 132 radially inward, thereby causing the multiple-turnspring 130 to contract, and allow the rotor 88 to turn, and thereforetransfer torque to the driven pinion gear 86 and thereafter to the powerscrew 46. The pinion gear 86 is axially slidable upon the end of therotor shaft 88, however, it has a snap ring (not shown) which connectsit thereto. The axial sliding movement is provided so that thermalexpansion will not cause the pinion gear 86 to make contact with thesleeve 120 or the motor housing 56. The pinion 86 has a flange 180 andthe spring 130 is axially captured between the flanges 110,180 of thedriver and the pinion. The pinion gear also has lobe torque transferportions 182 which project into the sleeve. The lobe sections arecontacted by the torque transfer portions 152 of the driver 100 totransfer torque between the members. However, the pinion gear typicallywill have two 182 lobes but only needs to have one. One pinion 182 lobemust provide a surface to make contact with one of the spring tangs onthe opposite end 135 of the tang that the contact surface 104 of thedriver makes contact on to cause spring tang 132 to expand outward tolock the motor 50. The locking action is provided in the direction fromthe piston's extreme position near its fluid connection with the wheelcylinder (As shown in FIG. 1). Therefore, from above-described extremeposition, rotating downward or in other word, retractive movement of thepiston 44 caused by fluid pressure within the longitudinal bore 32 willcause the lobe 182 on the pinion to hit the spring tang 132 on theopposite side 135 (FIG. 8) that the spring tang is hit by the drivercontact surface 104 causing the spring tang to be forced outwardexpanding the spring and preventing transfer torque from the power screw48 back to the motor rotor shaft 88 thereby locking the piston in itsextreme position within 3/100 of an inch from the top. Therefore, thecheck valve 38 is maintained in an open position. In most applicationlocking of the location of the piston 44 as from its lower extremeposition will not be relevant.

The spring 130 is configured in such a manner that once one of the rotorshaft 88 (or driver 100) contact surfaces 104 is in contact with thetang 132 the rotor only has to turn approximately 5 more to release thespring 130. The torque transfer portion 152 of the driver 100 will startturning the pinion 86 before the spring contact surface of the pinion(lobe) 182 can contact the tang 132 of the spring. Therefore, when themotor 50 is driving the piston 44 upward, there is isolation of thespring tang 182 from the pinion lobe 182 from a fraction of a degree toapproximately 5 degrees in a preferred embodiment of the inventionillustrated in FIGS. 3-5. In other words, tang isolation means that thetang is not sandwiched between the driver lobe 152 and the pinion lobe182.

First, the tang isolation is important in that when the motor 50 isdriving the piston 44 the rotor shaft 88 will start turning the pinion86 before making contact with the spring tang 132 and, therefore,torsional slip between the pinion 86 and the driver 100 will not cause achattering effect on the tang 132 of the spring 130 (the tang 132 is oneof the most critical components in this ABS system in regards toreliability) and, therefore, spring 130 life is preserved. Secondly,when the motor 50 drives the piston 44 to its up extreme position, thereis an impact force and the motor 50 will stall. However, instantaneouslythe actuator frame 30 will be loaded in tension. Instantaneously, thespring tension of the actuator frame 30 will cause a backward force orrearward force on the pinion gear 38, and the pinion gear will rotateslightly before the surface of its lobe 182 will come in contact withthe spring tang side 135. The spring isolation allows the pinion gear toback-lash a few degrees to relieve tension of the actuator frame 30before the pinion lobe locks up against the tang (lock up occurs in 1-5degrees). Therefore, the pinion 86 can reverse slightly (relievingtension in the actuator frame), however, not as much as to allow thecheck valve 38 to close. Without the tang isolation, the reversemovement of the pinion 86 would not be possible. Without tang isolationthe spring tang 132 will be instantly loaded and the piston 44 willremain in compression and in a position of tension loading the actuatorframe 28 which places the whole gear train 80 under a high retained loadin the non-ABS mode of operation (normal brake operation).

The spring contacting surface 104 of the driver is configured in such amanner as to impact a force to the tang normal to the generally straightportion 134. The rounded surface 104 provides almost point contact,imparting a force which also goes through the center of percussion 137of the tang 132. Therefore, vibration of the tang 132 with respect tothe driver contact surface 104 is minimized and tang 132 life isprolonged. The inward bend of less than 90 helps to prevent any tendencyof the tang to straighten out since the surface 136 is less than 90 fromthe tangent. In other words, there is a small component of forcedirected inwards which tends to pull the spring away from the sleeve. Ithas also been found to be preferable that the radius of the tang 132bend be in the neighborhood of 1 times the thickness of the wire of thespring 130 measured in the radial direction.

FIGS. 12 and 13 illustrate an alternative preferred embodiment of thepresent invention. FIG. 12 being a top elevational view and FIG. 13being a rolled out view of the alternative driver 200, spring 230, andgear 286. In the embodiment of FIG. 12 and 13 a first tang (not shown)which is generally most adjacent to the electric motor is as previouslydescribed. The gear 286 has a tab 282 with a generally radial hole 283typically shaped in a diverging or conical fashion with its largestdimension 285 radially inward. The second tang 232, as shown in FIG. 12,is inserted within the hole 283 of the nesting tab 282. The spring 230is now restrained in position by the gear and its snap ring (not shown)which results in several benefits. One advantage is that no flange isnow required on the driver to retain the spring axially within thenonrotative sleeve. Installation of the spring 230 is fairly easy andthe spring 230 can now be removed without removing the driver 200. Thegear nesting tab outer diameter 287 typically is raised radiallyoutwardly so that it contacts the second tang 232 near its knee 239without fear of it hitting an adjacent spring turn or missing contactaltogether with the tang 232. Therefore, bending stress in the secondspring tang 232 is greatly reduced. The nesting hole 283 of the gear tabprevents load stresses from bending the second spring tang beyond itsgenerally 90 degree bend and also prevents any tendency for the driver200 to unbend the tang 232 thus reducing the stress in the oppositedirection wherein the tang is being pushed to unlock the spring withinthe nonrotative sleeve. Stress reduction may be so great that it may bepossible to utilize round wire which is typically cheaper and easier towork with. Additionally, the tang is precisely located with thisapproach and the amount of overlap of the driver and gear tabs 252,282can be considerably minimized thus reducing driver and gear bendingtorques and shortening the overall assembly. In operation to release thetang 232 a tab 252 of the driver will make contact with the nesting tab282 of the gear and therefore transfer torsion from the driver 200 tothe gear 286 while at the same time causing the gear nesting tab 282 torelease the spring 230 from its locked condition causing a first side271 of the hole to make contact with the second spring tang 232. Torsiontransfer from the gear 286 to backdrive the driver 200 is preventedsince the opposite side 272 of the gear tab nesting hole 283 will hitthe tang 232 causing it to expand and lock up as mentioned in theprevious examples. Another advantage of the configuration is that in onedirection the driver 200 imparts force to the tang 232 through the geartab 282 and therefore the tang 232 is isolated from the driver tab(second contact surface) 204. Also, since the nesting tab 282 makescontact with the second tang 232 closer to the second tangs outerdiameter there is less stress induced upon the tang. As noted in FIG.12, the tang radius `R` 231 should be held to the minimum valuepossible.

Referring to the embodiment shown in FIGS. 14 and 15 there is addedjuxtaposed between the driver 300 and the gear 386 a nesting washer forthe second tang 332. The nesting washer 381 can rotate around the motorshaft 388 independent of the motion of the driver 300. The washer fitsloosely on the motor shaft 388 and as mentioned previously is capturedbetween the driver 300 and the gear 386. The washer has a raised portioncalled the washer tab 391 which is somewhat radially outward and withinthe tab 391 is a radial hole which captures the spring tang 332 (as in amanner similar to FIG. 12). The above allows the washer tab 391 tocontact the second tang 332 at a point closer to its knee 339 when thegear second contact surface 382 makes contact on the nesting washer 391to prevent the torque from the gear 386 being transferred to the driver300 via the driver torsion transfer portion 352. The driver firstcontact surface 304 engages the first spring tang 132 which is moreadjacent to the motor shaft 388 in a manner as previously described forthe previous embodiments, but for movement in the opposite seconddirection the driver torsion transfer portion 352 imparts movement tothe tang washer 381 to cause the tang washer 381 to move in a directionto impart force upon the second tang 332 in a direction to release thetang allowing torsional transfer from the driver 300 to the tang washer381 and to the gear torsion transfer portion 382 thereby allowing thedriver 300, tang washer 381 and gear 300 to move in unison.

This configuration provides several advantages. Again, no flanges areneeded on the driver for spring retention and installation is fairlyeasy. Again, the spring can be removed without removing the driver andthe other advantages are similar to that of the previously explainedembodiment. However, an additional advantage is that the washer nowallows independent movement of the gear with respect to the driver.Additionally when the gear 386 is imparting force to the second springtang 332 in a locking condition, the washer extension 391 allows contactwith the second spring tang 332 radially more outwardly, therebyproviding less of a tendency for the spring tang 332 to be bent inwardlythan when the gear contact surface 382 make direct contact with thesecond tang 332.

Referring to FIGS. 16 and 17, there is provided another version of thetang arrangement. As in prior versions the driver 400 makes contact withthe first tang 132 in a manner similar to that previously described.However, the second tang 432 is comprised of a generally radial portion431 which is connected with a loop portion 433 which loops around themotor shaft. This loop version has most of the advantages of the versionpreviously described with the tang washer but is further advantageous inthat it eliminates the tang washer. The generally radial portion 431 ofthe second tang 432 provides a surface for impartation of forces fromeither the driver contact surface 405 or the gear contact surface 482.And the loop version of the second tang also allows some independentmovement of the gear 486, the spring and driver 400 with respect to oneanother.

FIGS. 18 and 19 again illustrate alternative versions of the secondtang. In these versions the last partial turn of the spring 530 1630leading to the second tang has a slightly smaller inner diameter. In theversion of FIG. 19 there is provided a wedging nest upon a tab 581 ofthe gear 586 for making contact with the secondary spring tang 532. Thetang itself has a contracting mass formed from a 180 degree bend and iscontacted by a modified driver 500 second contact surface 504 which hasa blunt end which traps the tang 532 within the wedge nest 582 of thegear tab 581 to release the secondary tang and to power the gear 586.When the gear 586 is imparting force upon the second spring tang 532 tocause such tang 532 to lock within the nonrotative sleeve to preventtransfer of torque, the gear 586 imparts a force upon the spring 530which is almost tangential and therefore places less stress on the tang532 and more in a direction that is nearly ideal (tangential to thespring diameter). The `V` groove of the gear tab nest positions thesecond tang and prevents it from wedging from between the gear 586 andthe inside diameter of the spring sleeve 720. In the version of FIG. 18which is substantially similar to that shown in 19, the wire spring 630is abruptly bent and on it a welded-on short piece of wire 631contacting mass eliminates the requirement for the 180 degree bend atthe second tang end. This version will almost totally eliminate anytendency for bowing of the spring adjacent to the tang 632 when the gear686 is imparting force to the second tang.

FIGS. 20 and 21 illustrate a modification of the present invention moreakin to that shown in FIGS. 1 through 11 wherein the gear 786 iscomprised of two separate rigid pieces 785,787 which have juxtaposedbetween them in an elastomeric torsion spring member 780. The gearmember 786 with the teeth has two lugs 702 which are geometricallyopposed to two lugs 704 of the other gear member 785 which is connectedwith the contact surfaces 782 of the gear. Placed between these lugs andjoined by webbing 710 is the elastomeric torsion member 780. It has beenfound that a slight clearance between the torsion member 780 and thelugs 702,704 provides better torsional damping characteristics. Thesecharacteristics mainly come into play when the power screw 48 is drivingthe nut 46 to its extreme positions within the bore 32 of the actuator28. It has been found that utilization of this torsional damping systemsignificantly diminishes problems of strain within the system caused bythe nut 46 reaching an extreme position with respect to the bore 46while the motor 50 is still continuously running and therefore prolongsthe life of the overall anti-lock braking system.

While embodiments of the present invention have been explained, it willbe readily apparent to those skilled in the art of the variousmodifications which can be made to the present invention withoutdeparting from the spirit and scope of this application as it isencompassed by the following claims.

We claim:
 1. An anti-lock braking system for a wheel of an automotivevehicle including:master cylinder means for supplying pressurized fluid;sensor means to determine a rotational speed of the wheel; a wheel brakemeans receiving pressurized fluid from said master cylinder means andfor restraining rotational movement of said wheel; an anti-lock brakingcontroller cognizant of the rotational condition of said wheel via thesensor means and providing a signal when the rotational condition ofsaid wheel is within present parameters; an actuator frame having a borewith means of fluid communication with said wheel brake means; a pistonslidably sealably mounted with said bore for providing a variablecontrol volume in communication with said wheel brake means and therebymodulating the pressure therein; a nut operatively associated with saidpiston and slidably mounted within said bore in a no-rotative fashion; apower screw projecting into said nut and threadedly engaged within in alow friction, backdriveable manner, said power screw having a fixedrotational axis with respect to said actuator frame; reversible motormeans for powering said power screw, said motor means being responsiveto signals given by said controller; a non-rotative sleeve locatedbetween said power screw and said motor means in a fixed position, saidsleeve having an inner diameter of a first value; a coil spring having afree outer diameter of a second value generally equal to or greater thansaid first value, said coil spring being captured within saidnon-rotative sleeve, and said coil spring having a tang on each end; adriver member with a torsion transfer portion projecting into saidnon-rotative sleeve, said drive member having a rotational axis, saiddriver member having an angular position in correspondence wit theangular position of said motor means, said drive member having first andsecond contact surfaces for engagement with said spring tangs to causesaid tangs to release and to allow said spring to having slidingmovement within said non-rotative sleeve in both directions; a drivenmember with an angular position in correspondence with the angularposition of said power screw, said driven member having a rotationalaxis generally coterminous with said rotational axis of said drivermember and said driven member having projecting into said sleeve atorsion transfer portion for making contact with said driver membertorsion transfer portion for transferring torque from said driver memberto said power screw, said driven member having at least one contactsurface for making contact on said spring tang on the opposite side ofsaid spring tang said driver contact surface makes contact on to causesaid spring to radially expand and lock within said sleeve to preventtorque from said power screw being transferred to said motor means whensaid piston is moving away from a position more adjacent to saidactuator frame bore means of fluid communication with said wheel brakemeans by fluid pressure within bore wherein the improvement comprises:said driven member having first and second halves joined together withan elastomeric torsion member juxtaposed therebetween to absorbtorsional shocks from said nut reaching extreme positions within saidactuator bore.
 2. A braking system for a wheel of an automotive vehiclecomprising:master cylinder means for supplying pressurized fluid; sensormeans to determine a rotational speed of the wheel, a wheel brake meansreceiving pressurized fluid from said master cylinder means and forrestraining rotational movement of said wheel; a controller cognizant ofthe rotational condition of said wheel via the sensor means andproviding a signal when the rotational condition of said wheel is withinpresent parameters; an actuator frame having a bore with means of fluidcommunication with said wheel brake means; a piston slidably sealablymounted with said bore for providing a variable control volume incommunication with said wheel brake means and thereby modulating thepressure therein; a nut operatively associated with said piston andslidably mounted within said bore in a non-rotative fashion; a powerscrew projecting into said nut and threadedly engaged within a lowfriction, backdriveable manner, said power screw having a fixedrotational axis with respect to said actuator frame; reversible motormeans for powering said power screw, said motor means being responsiveto signals given by said controller; a non-rotative sleeve locatedbetween said power screw and said motor means in a fixed position, saidsleeve having an inner diameter of a first value; a coil spring having afree outer diameter of a second value generally equal to or greater thansaid first value, said coil spring being captured within saidnon-rotative sleeve, and said coil spring having a first tang on one endand a second tang on another end; a driver member with a torsiontransfer portion projecting into said non-rotative sleeve, said drivermember having a rotational axis, said drive member having an angularposition in correspondence with the angular position of said motormeans, said driver member having first and second contact surfaces forimparting force to said spring tangs to cause said tangs to release andto allow said spring to have sliding movement within said no-rotativesleeve in both directions; a driven member with an angular position incorrespondence with the angular position of said power screw, saiddriven member having a rotational axis generally coterminous with saidrotational axis of said driver member and said driven member havingprojecting into said sleeve a torsion transfer portion for makingcontact with said diver member torsion transfer portion for transferablyreceiving torque from said driver member to said power screw, saiddriven member having at least one contact surface for imparting force tosaid spring tang on the opposite side of said spring tang said drivercontact surface imparts force to cause said spring to radially expandand lock within said sleeve to prevent torque from said power screwbeing transferred to said motor means when said piston is moving awayfrom a position more adjacent to said actuator frame bore means of fluidcommunication with said wheel brake means by fluid pressure within thebore wherein the improvement comprises: said driven member having afirst and second halves joined together with an elastomeric torsionmember juxtaposed therebetween to absorb torsional shocks from said nutreaching extreme positions within said actuator bore.